1. Field of the Invention
The present invention relates to a sound image localization apparatus and is preferably applied to the case where a sound image reproduced with a headphone, for example, is localized at a given position.
2. Description of the Related Art
When an audio signal is supplied to a speaker and reproduced, a sound image is localized ahead of a listener. On the other hand, when the same audio signal is supplied to a headphone unit and reproduced, a sound image is localized within the listener's head, with the result that an extremely unnatural sound field is created.
In order to realize natural localization of a sound image in a headphone unit, there has been proposed a headphone unit adapted to enable, by measuring or calculating impulse responses from a given speaker position to both ears of a listener and by convoluting and reproducing audio signals with the impulse responses with the use of digital filters or the like, natural localization of a sound image outside the head as if the audio signals were reproduced from a real speaker (see Japanese Patent Laid-Open No. 2000-227350).
FIG. 1 shows the configuration of a headphone unit 100 for localizing a sound image of a one-channel audio signal outside the head. The headphone unit 100 digitally converts an analog audio signal SA of one channel inputted via an input terminal 1 by means of an analog/digital conversion circuit 2 to generate a digital audio signal SD, and supplies it to digital processing circuits 3L and 3R. The digital processing circuits 3L and 3R performs signal processing for localization outside the head, on the digital audio signal SD.
As shown in FIG. 2, when a sound source SP at which the sound image is to be localized is located in front of a listener M, a sound outputted from the sound source SP reaches the left and right ears of the listener M via paths with transfer functions HL and HR. The impulse responses of the left and right channels with the transfer functions HL and HR converted to time axes are measured or calculated in advance.
The digital processing circuits 3L and 3R convolute the digital audio signal SD with the above-described left-channel and right-channel impulse responses, respectively, and outputs the obtained signals as digital audio signals SDL and SDR. The digital processing circuits 3L and 3R are configured by an Finite Impulse Response (FIR) filter as shown in FIG. 3.
Digital/analog conversion circuits 4L and 4R analogously convert the digital audio signals SDL and SDR to generate analog audio signals SAL and SAR, respectively, amplify the analog audio signals with corresponding amplifiers 5L and 5R and supply them to a headphone 6. Acoustic units (electric/acoustic conversion devices) 6L and 6R of the headphone 6 convert the analog audio signals SAL and SAR to sounds, respectively, and output the sounds.
Accordingly, the left and right reproduced sounds outputted from the headphone 6 are equivalent to the sounds which have reached from a sound source SP shown in FIG. 2 via the paths with the transfer functions HL and HR. Thereby, when the listener equipped with the headphone 6 listens to the reproduced sounds, the sound image is localized at the position of the sound source SP shown in FIG. 2 (namely, outside the head).
Description has been made on the case of one sound image. Next, description will be made on the case where multiple sound images are localized at different sound source positions.
Description will be made with the use of FIG. 3 on a headphone unit 101 in the case of localizing a sound image at each of two positions of a forward sound source SPf straight ahead of a listener and an upper sound source SPu α° above and ahead of the listener as shown in FIG. 4, for example. Impulse responses of transfer functions HfL and HfR from the forward sound source SPf to both ears of the listener M and transfer functions HuL and HuR from the upper sound source SPu to both ears of the listener M converted to time axes are measured or calculated in advance.
In FIG. 5, an analog/digital conversion circuit 2f of the headphone unit 101 digitally converts an analog audio signal SAf for front localization inputted via an input terminal 1f to generate a digital audio signal SDf, and supplies it to subsequent-stage digital processing circuits 3fL and 3fR. Similarly, an analog/digital conversion circuit 2u digitally converts an analog audio signal SAu for upper localization inputted via an input terminal 1u to generate a digital audio signal SDu, and supplies it to subsequent-stage digital processing circuits 3uL and 3uR.
The digital processing circuits 3fL and 3uL convolute digital audio signals SDf and SDu with impulse responses to the left ear, respectively, and supply the digital audio signals to an addition circuit 7L as digital audio signals SDfL and SDuL. Similarly, the digital processing circuits 3fR and 3uR convolute digital audio signals SDf and SDu with impulse responses to the right ear, respectively, and supply the signals to the addition circuit 7R as digital audio signals SDfR and SDuR. Each of the digital processing circuits 3fL, 3fR, 3uL and 3uR is configured by the FIR filter shown in FIG. 3.
The addition circuit 7L adds the digital audio signals SDfL and SDuL convoluted with the impulse responses, to generate a left-channel digital audio signal SDL. Similarly, the addition circuit 7R adds the digital audio signals SDfR and SDuR convoluted with the impulse responses, to generate a right-channel digital audio signal SDR.
The digital/analog conversion circuits 4L and 4R analogously convert the digital audio signals SDL and SDR to generate analog audio signals SAL and SAR, respectively, amplify the analog audio signals with the corresponding amplifiers 5L and 5R and supply them to the headphone 6. The acoustic units 6L and 6R of the headphone 6 convert the analog audio signals SAL and SAR to sounds, respectively, and output the sounds.
Left and right reproduced sounds outputted from the headphone 6 are equivalent to sounds which have reached from the forward sound source SPf shown in FIG. 4 via the paths with the transfer functions HfL and HfR, and equivalent to sounds which have reached from the upper sound source SPu via the paths with the transfer functions HuL and HuR, respectively. Thereby, when the listener equipped with the headphone 6 listens to the reproduced sounds, sound images are localized at the positions of the forward sound source SPf and the upper sound source SPu.